There are three basic CVD methods of producing Si.sub.3 N.sub.4 films on substrates: The thermal method, the plasma method, and the photo-excitation method. However, each presents serious problems that have prevented widespread use of Si.sub.3 N.sub.4 films as insulating or protective layers, particularly in the semiconductor and integrated circuit fields.
The Thermal Method.
This method involves thermal reaction of a gas mixture containing SiH.sub.4 and ammonia at high temeratures, typically 700.degree.-1000.degree. C. This process has not been used in the semiconductor field because transistors cannot be subjected to the 700.degree. C. and higher temperatures without seriously affecting or destroying their performance capabilities to a point where production output is uneconomic.
Typical of thermal processes is that shown by Rosler et al. in U.S. Pat. No. 4,232,063, wherein the reaction of monosilane (SiH.sub.4) with ammonia at 800.degree.-900.degree. C. forms Si.sub.3 N.sub.4 in a hot-wall reactor in which the wafers are placed vertically on a quartz wafer carrier.
In Hirai et al. U.S. Pat. No. 4,118,539, a super hard, crystalline Si.sub.3 N.sub.4 coating is produced on a heated substrate by a thermal CVD process at 500.degree.-1900.degree. C., preferably 1000.degree.-1600.degree. C. A concentric pipe assembly blows individual streams of gases onto the heated substrate, with the nitrogen source gas being surrounded by the silicon source. Silicon halides, silicon hydrides (including Si.sub.2 H.sub.6), and silicon hydrogen hydrides are reacted with nitrogen hydrides (including ammonia) and ammonium halides.
Hirai et al. also gives a description of several prior art thermal CVD processes circa the mid-1970's involving the use of monosilane or silicon halides plus ammonia at substrate temperatures of 400.degree. C. and above.
The Plasma Method.
In attempts to achieve lower temperatures, plasma CVD methods have been tried wherein a discharge plasma is applied to the reaction gases in the region close to the substrate surface. The plasma method, while cooler than 700.degree. C., generates serious radiation damage. For example, the radiation field causes metals to migrate, the reaction stoichiometry cannot be controlled, the film thickness and integrity is not uniform or satisfactory, and by-products are introduced into the film. In addition, the dissipation of stored charges by the radiation field prevents any use for MOS devices.
Nitrogen trifluoride gas is incidentally mentioned, among other compounds, in a series of Canon Company plasma method patents as a way to introduce nitrogen atoms into a silicon matrix layer. The matrix layer is used as an adhesion layer between a substrate and a photoconductive layer for copier drums. These plasma method patents and the mention of NF.sub.3 are: Ogawa et al. U.S. Pat. No. 4,452,875 (col. 6, lines 13-24); Shimizu et al. U.S. Pat. No. 4,394,426 (col. 16, lines 13-20); Kanbe et al. U.S. Pat. No. 4,420,546 (col. 6, lines 20-32); Shirai et al. (Shirai I) U.S. Pat. No. 4,405,702 (col. 9, lines 9-26); Shirai et al. (Shirai II) U.S. Pat. No. 4,461,820 (col. 8, lines 38-50); and Shirai et al. (Shirai III) U.S. Pat. No. 4,464,451 (col. 7, line 68, col. 8, lines 1-5; and col. 23, lines 36-46).
The Ogawa et al. U.S. Pat. No. 4,452,875 of Canon is directed to a photosensitive copier drum in which one or more interface layers of an amorphous Si-containing material are provided. These interface layers are primarily for the purpose of enhancement of adhesion between a metal support such as an aluminum copier drum, and an overlying rectifying layer which functions primarily for preventing migration of charges from the aluminum drum into photosensitive amorphous layer(s) overlying the reactifying layer. The interface layer is coated directly onto the drum. It is covered by an overlay of the rectifying layer and at least two amorphous overcoating layers. An additional interface layer may be interposed between the rectifying layer and the two photosensitive amorphous overcoating layers. The upper amorphous layer is a sealer which provides humidity resistance, abrasion resistance, dielectric strength and environmental characteristics in use and durability. It protects the underlying first amorphous layer which is the photosensitive layer of the copier drum.
There is omnibus disclosure in columns 5-6 of the Ogawa et al. patent directed to a list of a number of silanes, (monosilane, disilane, trisilane and tetrasilane) as a gas for supplying silicon (col. 5, lines 39-45). Nitrogen atoms are introduced into the interface layer which is described as an amorphous material containing silicon atoms as a matrix and nitrogen atoms, if desired, together with at least one member of the group of hydrogen atoms and halogen atoms as constituent atoms. The matrix is designated as "a-SIN(H,X)", meaning an amorphous silicon matrix containing nitrogen, and optionally hydrogen and halogen, as constituent atoms. The patent lists in an omnibus disclosure the folllwing as a starting gas for introduction of nitrogen in the amorphous silicon interface layer matrix: nitrogen, ammonia, hydrazine, hydrogen azide, ammonium azide, nitrogen trifluoride, and nitrogen tetrafluoride (col. 6, lines 13-24).
The interface layer is applied to the drum by a plasma method (glow discharge and sputtering in combination), which involves an electrode to which high power is applied and on which are placed high purity silicon nitride wafers, at a desired sputter area ratio, as targets. The heated support is spaced from the sputter targets on the electrode and the space between is evacuated. In the actual examples, Ogawa et al. discloses only nitrogen gas or ammonia gas to provide the nitrogen atom dopant for the silicon and silicon nitride powders sputtered onto the support (aluminum drum) surface.
In the Ogawa et al. examples, only monosilane and ammonia in a ratio of 1 to 30 is used in the sputtering method. This is not a proper stoichiometric ratio for formation of silicon nitride under the plasma discharge conditions shown in the Ogawa et al. patent. Thus, Ogawa et al., in spite of the incidental mention in an omnibus plasma method disclosure of disilane and nitrogen trifluoride, does not teach a process for producing silicon nitride films in a thermal low temperature CVD process. Rather, they dope a matrix of silicon and silicon nitride with nitrogen, hydrogen and halogen by using a gas atmosphere in a high voltage plasma process. The resulting Ogawa et al. film is employed as an adhesion layer directly on a metal substrate.
The Photo-Excitation Method.
A recently reported development is a high energy photo excitation method as described in Azuma et al. U.S. Pat. No. 4,495,218. This method uses ultraviolet radiation as an energy source for pyrolysis of a polysilane, alone to produce an Si film, with O.sub.2 or an oxygen-containing reactive gas to produce an SiO.sub.2 film, or with ammonia to produce Si.sub.3 N.sub.4. While the reaction temperature is reported to be in the range of 20.degree.-300.degree. C., the process has the disadvantage of requiring the presence of a small vessel of mercury heated to 30.degree.-40.degree. C. The UV light source disclosed in the examples was either a low pressure mercury lamp or an excimer laser (KrF, 249 nm). In the case of preparing Si.sub.3 N.sub.4 by the reaction of disilane with ammonia, the film forming rate was 150 Angstroms/min.
Another example of laser-induced photochemical process is shown in U.S. Pat. No. 4,227,907 of Merritt, wherein a silicon optical fiber, freshly drawn at about 2000.degree. C., is immediately hermetically sealed by cladding with Si.sub.3 N.sub.4 by passing the freshly drawn fiber through a chamber having a controlled atmosphere of SiX.sub.4 and NX.sub.3, wherein X is H and/or F, irradiated with a CO.sub.2 laser.
In related U.S. Pat. No. 4,270,997, Merritt also discloses the preparation of bulk powdered Si.sub.3 N.sub.4 at room temperature by reaction of SiH.sub.4 plus NF.sub.3 by the laser photochemical technique.
Other References.
Nitrogen trifluoride has also been used as a source of nitrogen for preparing nitrogen-doped silica glass as disclosed in Edahiro et al. U.S. Pat. No. 4,402,720 (col. 4, lines 28-34).
Accordingly, there is a great need in the art for a low temperature thermal CVD process that does not have the disadvantages of the high temperature process or the high energy plasma and photo-excitation processes.